Electrophotographic image formation comprises uniform charging of the surface of a photoreceptor drum or belt; exposure of the photoreceptor surface to light and formation on the photoreceptor surface of a charge pattern, i.e., a latent image, that mirrors the information to be transferred into a real image; developing the latent image with electrostatically charged toner particles comprising a colorant dispersed in a binder resin; transferring the developed toner onto a substrate, e.g. paper; fusing the image onto a substrate; and preparing the photoreceptor surface for the next cycle by erasing the residual electrostatic charges and cleaning the remaining toner particles.
Toners for use in electrophotography and electrostatic printing include a binder resin and a colorant, and may further include a charge control agent, an offset-preventing agent, and other additives. External toner additives such as metal oxide particles are often combined with toner particles in order to improve selected properties of the toner particles, including fluidity, transferability, fixability, and cleaning properties. Typically, the metal oxide particles, e.g., silica, alumina, or titania, are subjected to a chemical treatment to render the surface of the metal oxide particles hydrophobic. In addition, the metal oxide particles strongly influence the chargeability, i.e., tribocharge, of the toner composition. For example, toner containing silica as an additive exhibits higher absolute levels of tribocharge than toner containing titania. However, the tribocharge of silica is sensitive to humidity conditions. Such a dependence of the tribochargeability on environmental conditions leads to impaired transferability of the image and ultimately to reduced image quality.
Thus, it is desirable to have an external toner additive that exhibits high tribocharge that is stable with respect to environmental conditions.